Method, Apparatus and Computer Program Product for Read Before Programming Process on Programmable Resistive Memory Cell

ABSTRACT

A method, system and computer program product for programming a plurality of programmable resistive memory cells is disclosed. The method comprises executing the following for each memory cell: reading a resistance of a memory cell and reading input data corresponding to the memory cell. The method further comprises executing the following for each memory cell: programming the memory cell to a lower resistance (SET) state if the resistance is at a higher resistance state and the input data corresponds to a first (SET) state and programming the memory cell to a higher resistance (RESET) state if the resistance is at a lower resistance state and the input data corresponds to a second (RESET) state.

CROSS-REFERENCE TO OTHER APPLICATIONS

The subject matter of this patent application is related to the subjectmatter of the following U.S. patent applications of the same inventor,filed on the same day as this application and assigned to the sameassignee: Ser. No. ______, filed on ______, 2006, entitled “Method,Apparatus and Computer Program Product for Read Before ProgrammingProcess on Multiple Programmable Resistive Memory Cells,” AttorneyDocket MXIC 1773-1, and U.S. patent application Ser. No. ______ filed on______, 2006, entitled “Method, Apparatus and Computer Program Productfor Stepped Reset Programming Process on Programmable Resistive MemoryCell,” Attorney Docket MXIC 1776-1.

PARTIES TO A JOINT RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation;Macronix International Corporation, Ltd., a Taiwan corporation, andInfineon Technologies A. G., a German corporation, are parties to aJoint Research Agreement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high density memory devices based onmemory materials, for example resistor random access memory (RRAM)devices, and to methods for programming such devices. The memorymaterial is switchable between electrical property states by theapplication of energy. The memory materials may be phase change basedmemory materials, including chalcogenide based materials, and othermaterials.

2. Description of Related Art

Phase change based memory materials are widely used in read-writeoptical disks. These materials have at least two solid phases, includingfor example a generally amorphous solid phase and a generallycrystalline solid phase. Laser pulses are used in read-write opticaldisks to switch between phases and to read the optical properties of thematerial after the phase change.

Phase change based memory materials, like chalcogenide based materialsand similar materials, also can be caused to change phase by applicationof electrical current at levels suitable for implementation inintegrated circuits. The generally amorphous state is characterized byhigher resistivity than the generally crystalline state; this differencein resistance can be readily sensed to indicate data. These propertieshave generated interest in using programmable resistive material to formnonvolatile memory circuits, which can be read and written with randomaccess.

In phase change memory, data is stored by causing transitions betweenamorphous and crystalline states in the phase change material usingcurrent. Current heats the material and causes transitions between thestates. The change from the amorphous to the crystalline state isgenerally a lower current operation. The change from crystalline toamorphous, referred to as reset herein, is generally a higher currentoperation, which includes a short high current density pulse to melt orbreakdown the crystalline structure, after which the phase changematerial cools quickly, quenching the phase change process, allowing atleast a portion of the phase change structure to stabilize in theamorphous state.

Each memory cell of a phase change memory device is coupled to a bitline and an access device, such as a transistor, wherein the accessdevice is coupled to a word line. The method by which the resistance ofa phase change memory cell is read, set or reset involves theapplication of bias voltages to the bit line and word line for thememory cell. In order to apply a set voltage pulse or a reset voltagepulse to a phase change memory cell, the word and bit lines must beconnected to circuitry providing the set voltage pulse or the resetvoltage pulse. The creation of these connections for setting orresetting a phase change memory cell is referred to as “bit line set up”and “word line set up.” There is a time and resource expenditureassociated with the steps taken during bit line setup and word linesetup. Therefore, there is a desire to reduce the number of steps takenduring bit line setup and word line setup. Furthermore, when handlingthe programming of successive phase change memory cells in an array ofphase change memory cells, word line setup and bit line setup for afirst memory cell may necessitate a change if the set/reset programmingfor a memory cell is different from the set/reset programming of theimmediately preceding memory cell. Changing a word line setup or a bitline setup when sequentially programming memory cells also expends timeand resources. Therefore, there is a further desire to reduce the numberof times a word line setup or a bit line setup is changed whenprogramming successive phase change memory cells in an array of phasechange memory cells.

Accordingly, an opportunity arises to devise methods and structures thatreduce the steps taken in a bit line setup and a word line setup whenprogramming a phase change memory cell or programming successive phasechange memory cells in an array of phase change memory cells.

BRIEF SUMMARY OF THE INVENTION

A first aspect of the present invention relates to a method forprogramming a plurality of programmable resistive memory cells. Oneexemplary method comprises executing the following for each memory cell:reading a resistance of a memory cell and reading input datacorresponding to the memory cell. The method further comprises executingthe following for each memory cell: programming the memory cell to alower resistance (SET) state if the resistance is at a higher resistancestate and the input data corresponds to a first (SET) state, andprogramming the memory cell to a higher resistance (RESET) state if theresistance is at a lower resistance state and the input data correspondsto a second (RESET) state.

A second aspect of the present invention relates to a programmableresistive memory system that in one example comprises a memory celldevice comprising a plurality of programmable resistive memory cells anda controller for programming the memory cell device. The controller isconfigured for executing the following for each memory cell: reading aresistance of a memory cell and reading input data corresponding to thememory cell. The controller is further configured for executing thefollowing for each memory cell: programming the memory cell to a lowerresistance (SET) state if the resistance is at a higher resistance stateand the input data corresponds to a first (SET) state and programmingthe memory cell to a higher resistance (RESET) state if the resistanceis at a lower resistance state and the input data corresponds to asecond (ESET) state.

A third aspect of the present invention relates to a computer programproduct including computer instructions for programming a memory celldevice event in one example comprises a plurality of programmableresistive memory cells. The computer instructions include instructionsfor executing the following for each memory cell: reading a resistanceof a memory cell and reading input data corresponding to the memorycell. The computer instructions further include instructions forexecuting the following for each memory cell: programming the memorycell to a lower resistance (SET) state if the resistance is at a higherresistance state and the input data corresponds to a first (SET) stateand programming the memory cell to a higher resistance (RESET) state ifthe resistance is at a lower resistance state and the input datacorresponds to a second (RESET) state.

Various features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an integrated circuit device in accordancewith the present invention.

FIG. 2 is a partial schematic diagram of a representative memory arrayas shown in FIG. 1.

FIG. 3 is a graph of voltage and temperature versus time for pulses usedfor programming programmable resistive memory cells, according to oneembodiment of the present invention.

FIG. 4 is a flowchart showing the control flow of a general process forprogramming a single memory cell comprising a phase change material, inaccordance with one embodiment of the present invention.

FIG. 5 is a flowchart showing the control flow of the process forprogramming a single memory cell comprising a phase change material, inaccordance with one embodiment of the present invention.

FIG. 6 is a flowchart showing the control flow of the process forsequentially programming a plurality of memory cells comprising a phasechange material, in accordance with one embodiment of the presentinvention.

FIG. 7 is a flowchart showing the control flow of the process for groupprogramming of a plurality of memory cells comprising a phase changematerial, in accordance with one embodiment of the present invention.

FIG. 8 is a flowchart showing the control flow of the process for groupprogramming of a plurality of memory cells comprising a phase changematerial, in accordance with another embodiment of the presentinvention.

FIG. 9 is a flowchart showing the control flow of the group set processfor programming a plurality of memory cells comprising a phase changematerial, in accordance with one embodiment of the present invention.

FIG. 10 is a flowchart showing the control flow of the group resetprocess for programming a plurality of memory cells comprising a phasechange material, in accordance with one embodiment of the presentinvention.

FIG, 11 is a flowchart showing the control flow of the process for resetprogramming a memory cell comprising a phase change material, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the invention will typically be withreference to specific structural embodiments and methods. It is to beunderstood that there is no intention to limit the invention to thespecifically disclosed embodiments and methods but that the inventionmay be practiced using other features, elements, methods andembodiments. Preferred embodiments are described to illustrate thepresent invention, not to limit its scope, which is defined by theclaims. Those of ordinary skill in the art will recognize a variety ofequivalent variations on the description that follows. Like elements invarious embodiments are commonly referred to with like referencenumerals.

Referring to FIG. 1, shown is a simplified block diagram of anintegrated circuit 10 in which the present invention may be implemented.Circuit 10 includes a memory array 12 implemented using phase changememory cells (not shown) on a semiconductor substrate, discussed morefully below. A word line decoder 14 is in electrical communication witha plurality of word lines 16. A bit line decoder 18 is in electricalcommunication with a plurality of bit lines 20 to read data from, andwrite data to, the phase change memory cells (not shown) in array 12.Addresses are supplied on bus 22 to word line decoder and drivers 14 andbit line decoder 18. Sense amplifiers and data-in structures in block 24are coupled to bit line decoder 18 via data bus 26. Data is suppliedfrom an input buffer 27 via a data-in line 28 from input/output ports onintegrated circuit 10, or from other data sources internal or externalto integrated circuit 10, to data-in structures in block 24. Othercircuitry 30 may be included on integrated circuit 10, such as a generalpurpose processor or special purpose application circuitry, or acombination of modules providing system-on-a-chip functionalitysupported by array 12. Data is supplied via a data-out line 32 from thesense amplifiers in block 24 to input/output ports on integrated circuit10, or to other data destinations internal or external to integratedcircuit 10.

A controller 34 implemented in this example, using a bias arrangementstate machine, controls the application of bias arrangement supplyvoltages 36, such as read, program, erase, erase verify and programverify voltages. Controller 34 may be implemented using special-purposelogic circuitry as known in the art. In alternative embodiments,controller 34 comprises a general-purpose processor, which may beimplemented on the same integrated circuit to execute a computer programto control the operations of the device. In yet other embodiments, acombination of special-purpose logic circuitry and a general-purposeprocessor may be utilized for implementation of controller 34.

As shown in FIG. 2 each of the memory cells of array 12 includes anaccess transistor (or other access device such as a diode), four ofwhich are shown as 38, 40, 42 and 44, and a memory element, typically aphase change element, shown as 46, 48, 50 and 52. Sources of each ofaccess transistors 38, 40, 42 and 44 are connected in common to a sourceline 54 that terminates in a source line termination 55. In anotherembodiment the source lines of the select devices are not electricallyconnected, but independently controllable. A plurality 16 of word linesincluding word lines 56 and 58 extend parallel along a first direction.Word lines 56 and 58 are in electrical communication with word linedecoder 14. The gates of access transistors 38 and 42 are connected to acommon word line, such as word line 56, and the gates of accesstransistors 40 and 44 are connected in common to word line 58. Aplurality 20 of bit lines including bit lines 60 and 62 have one end ofphase change elements 46 and 48 connected to bit line 60 via separateconnections 88. Specifically, phase change element 46 is connectedbetween the drain of access transistor 38 and bit line 60, and phasechange element 48 is connected between the drain of access transistor 40and bit line 60. Similarly, phase change element 50 is connected betweenthe drain of access transistor 42 and bit line 62, and phase changeelement 52 is connected between the drain of access transistor 44 andbit line 62. It should be noted that four memory cells are shown forconvenience of discussion and in practice array 12 may comprisethousands to millions of such memory cells. Also, other ray structuresmay be used, e.g. the phase change memory element is connected tosource.

Useful characteristics of a programmable resistive type of memorymaterial, like a phase change material, include the material having aresistance which is programmable, and preferably in a reversible manner,such as by having at least two solid phases that can be reversiblyinduced by electrical current. These at least two phases include anamorphous phase and a crystalline phase. However, in operation, theprogrammable resistive material may not be fully converted to either anamorphous or crystalline phase. Intermediate phases or mixtures ofphases may have a detectable difference in material characteristics. Thetwo solid phases should generally be bistable and have differentelectrical properties. The programmable resistive material may be achalcogenide material. A chalcogenide material may include GST. Infollowing sections of the disclosure, the phase change or other memorymaterial is often referred to as GST, and it will be understood thatother types of phase change materials can be used. A material useful forimplementation of a memory cell as described herein is Ge₂Sb₂Te₅.

A memory cell device 10 as described herein is readily manufacturableusing standard lithography and thin film deposition technologies,without requiring extraordinary steps to form sub-lithographic patterns,while achieving very small dimensions for the region of the cell thatactually changes resistivity during programming. In embodiments of theinvention, the memory material may be a programmable resistive material,typically a phase change material, such as Ge₂Sb₂Te₅ or other materialsdescribed below. The region in the memory element 16 that changes phaseis small; and accordingly, the magnitude of the reset current requiredfor changing the phase is very small.

Embodiments of memory cell device 10 include phase change based memorymaterials, including chalcogenide based materials and other materials,for memory element 16. Chalcogens include any of the four elementsoxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming partof group VI of the periodic table. Chalcogenides comprise compounds of achalcogen with a more electropositive element or radical. Chalcogenidealloys comprise combinations of chalcogenides with other materials suchas transition metals. A chalcogenide alloy usually contains one or moreelements from column six of the periodic table of elements, such asgermanium (Ge) and tin (Sn). Often, chalcogenide alloys includecombinations including one or more of antimony (Sb), gallium (Ga),indium (In), and silver (Ag). Many phase change based memory materialshave been described in technical literature, including alloys of: Ga/Sb,In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te,In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In thefamily of Ge/Sb/Te alloys, a wide range of alloy compositions may beworkable. The compositions can be characterized asTe_(a)Ge_(b)Sb_(100-(a+b)), where a and b represent atomic percentagesthat total 100% of the atoms of the constituent elements. One researcherhas described the most useful alloys as having an average concentrationof Te in the deposited materials well below 70%, typically below about60% and ranged in general from as low as about 23% up to about 58% Teand most preferably about 48% to 58% Te. Concentrations of Ge were aboveabout 5% and ranged from a low of about 8% to about 30% average in thematerial, remaining generally below 50%. Most preferably, concentrationsof Ge ranged from about 8% to about 40%. The remainder of the principalconstituent elements in this composition was Sb. (Ovshinsky '112 patent,cols 10-11.) Particular alloys evaluated by another researcher includeGe₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇. (Noboru Yamada, “Potential of Ge-Sb-TePhase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109,pp. 28-37 (1997).) More generally, a transition metal such as chromium(Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum(Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te toform a phase change alloy that has programmable resistive properties.Specific examples of memory materials that may be useful are given inOvshinsky '112 at columns 11-13, which examples are hereby incorporatedby reference.

Phase change materials are capable of being switched between a firststructural state in which the material is in a generally amorphous solidphase, and a second structural state in which the material is in agenerally crystalline solid phase in its local order in the activechannel region of the cell. These phase change materials are at leastbistable. The term amorphous is used to refer to a relatively lessordered structure, more disordered than a single crystal, which has thedetectable characteristics such as higher electrical resistivity thanthe crystalline phase. The term crystalline is used to refer to arelatively more ordered structure, more ordered than in an amorphousstructure, which has detectable characteristics such as lower electricalresistivity than the amorphous phase. Typically, phase change materialsmay be electrically switched between different detectable states oflocal order across the spectrum between completely amorphous andcompletely crystalline states. Other material characteristics affectedby the change between amorphous and crystalline phases include atomicorder, free electron density and activation energy. The material may beswitched either into different solid phases or into mixtures of two ormore solid phases, providing a gray scale between completely amorphousand completely crystalline states The electrical properties in thematerial may vary accordingly,

Phase change materials can be changed from one phase state to another byapplication of electrical pulses. It has been observed that a shorter,higher amplitude pulse tends to change the phase change material to agenerally amorphous state, and is referred to as a reset pulse. Alonger, lower amplitude pulse tends to change the phase change materialto a generally crystalline state, and is referred to as a program pulse.The energy in a shorter, higher amplitude pulse is high enough to allowfor bonds of the crystalline structure to be broken and short enough toprevent the atoms from realigning into a crystalline state. Appropriateprofiles for pulses can be determined empirically, without undueexperimentation, specifically adapted to a particular phase changematerial and device structure.

The following are short summaries describing four types of resistivememory materials.

1. Chalcogenide material

-   -   Ge_(x)Sb_(y)Te_(z)    -   x:y:z=2:2:5    -   Or other compositions with x: 0˜5; y: 0 5; z: 0˜10    -   GeSbTe with doping, such as N—, Si—, Ti—, or other element        doping may also be used.

Formation method: By PVD sputtering or magnetron-sputtering method withreactive gases of Ar, N₂, and/or He, etc chalcogenide @ the pressure of1 mtorr 100 mtorr. The deposition is usually done at room temperature.The collimator with aspect ratio of 1˜5 can be used to improve thefill-in performance. To improve the fill-in performance, the DC bias ofseveral ten to several hundred volts is also used. On the other hand,the combination of DC bias and the collimator can be usedsimultaneously.

The post deposition annealing treatment with vacuum or N2 ambient issometimes needed to improve the crystallize state of chalcogenidematerial. The annealing temperature typically ranges 100 C to 400 C withan anneal time of less than 30 minutes.

The thickness of chalcogenide material depends on the design of cellstructure. In general, a chalcogenide material with thickness of higherthan 8 nm can have a phase change characterization so that the materialexhibits at least two stable resistance states.

2. CMR (colossal magneto resistance) material

-   -   Pr_(x)Ca_(y)MnO₃    -   x:y=0.5:0.5    -   Or other compositions with x: 0˜1; y: 0˜1    -   Another CMR material that includes Mn oxide may be used

Formation method: By PVD sputtering or magnetron-sputtering method withreactive gases of Ar, N₂, O₂, and/or He, etc. at the pressure of 1 mtorr100 mtorr. The deposition temperature can range from room temperature to˜600 C, depending on the post deposition treatment condition. Acollimator with an aspect ratio of 1˜5 can be used to improve thefill-in performance. To improve the fill-in performance, the DC bias ofseveral ten to several hundred volts is also used. On the other hand,the combination of DC bias and the collimator can be usedsimultaneously. A magnetic field of several ten gauss to 10,000 gaussmay be applied to improve the magnetic crystallized phase.

The post deposition annealing treatment with vacuum or N₂ ambient orO₂/N₂ mixed ambient may be needed to improve the crystallized state ofCMR material. The annealing temperature typically ranges 400 C to 600 Cwith an anneal time of less than 2 hours.

The thickness of CMR material depends on the design of cell structure.The CMR thickness of 10 nm to 200 nm can be used to be the corematerial.

A buffer layer of YBCO (YBaCuO3, a kind of high temperaturesuperconductor material) is often used to improve the crystallized stateof CMR material. The YBCO is deposited before the deposition of CMRmaterial. The thickness of YBCO ranges 30 nm to 200 nm.

3.2-element compound

-   -   Ni_(x)O_(y); Ti_(x)O_(y); Al_(x)O_(y); W_(x)O_(y); Zn_(x)O_(y);        Zr_(x)O_(y); Cu_(x)O_(y); etc    -   x:y=0.5:0.5    -   Other compositions with x: 0˜1; y: 0˜1    -   Formation method:

A. Deposition: By PVD sputtering or magnetron-sputtering method withreactive gases of Ar, N₂, O₂, and/or He, etc. at the pressure of 1 mtorr100 mtorr, using a target of metal oxide, such as Ni_(x)O_(y);Ti_(x)O_(y); Al_(x)O_(y); W_(x)O_(y); Zn_(x)O_(y); Zr_(x)O_(y);C_(x)O_(y); etc. The deposition is usually done at room temperature. Acollimator with an aspect ratio of 1˜5 can be used to improve thefill-in performance. To improve the fill-in performance, the DC bias ofseveral ten to several hundred volts is also used. If desired, theycombination of DC bias and the collimator can be used simultaneously.

The post deposition annealing treatment with vacuum or N₂ ambient orO₂/N₂ mixed ambient as sometimes needed to improve the oxygendistribution of metal oxide. The annealing temperature ranges 400 C to600 C with an anneal time of less than 2 hours.

B. Reactive deposition: By PVD sputtering or magnetron-sputtering methodwith reactive gases of Ar/O₂, Ar/N₂/O₂, pure O₂, He/O₂ He/N₂/O₂ etc. atthe pressure of 1 mtorr 100 mtorr, using a target of metal oxide, suchas Ni, Ti, Al, W, Zn, Zr, or Cu etc. The deposition is usually done atroom temperature. A collimator with an aspect ratio of 1˜5 can be usedto improve the fill-in performance. To improve the fill-in performance,a DC bias of several ten to several hundred volts is also used. Ifdesired, the combination of DC bias and the collimator can be usedsimultaneously.

The post deposition annealing treatment with vacuum or N₂ ambient orO₂/N₂ mixed ambient is sometimes needed to improve the oxygendistribution of metal oxide. The annealing temperature ranges 400 C to600 C with an anneal time of less than 2 hours.

C. Oxidation: By a high temperature oxidation system, such as furnace orRTP system. The temperature ranges from 200 C to 700 C with pure O₂ orN₂/O₂ mixed gas at a pressure of several mtorr to 1 atm. The time canrange several minute to hours. Another oxidation method is plasmaoxidation. An RF or a DC source plasma with pure O₂ or Ar/O₂ mixed gasor Ar/N₂/O₂ mixed gas at a pressure of 1 mtorr to 100 mtorr is used tooxidize the surface of metal, such as Ni, Ti, Al, W, Zn, Zr, or Cu etc.The oxidation time ranges several seconds to several minutes. Theoxidation temperature ranges room temperature to 300 C, depending on thedegree of plasma oxidation.

4. Polymer material

-   -   TCNQ with doping of Cu, C₆₀, Ag etc.    -   PCBM-TCNQ mixed polymer    -   Formation method:

A. Evaporation: By thermal evaporation, e-beam evaporation, or molecularbeam epitaxy (MBE) system. A solid-state TCNQ and dopant pellets areco-evaporated in a single chamber. The solid-state TCNQ and dopantpellets are put in a W-boat or a Ta-boat or a ceramic boat. A highelectrical current or an electron-beam is applied to melt the source sothat the materials are mixed and deposited on wafers. There are noreactive chemistries or gases. The deposition is done at a pressure of10-4 torr to 10-10 torr. The wafer temperature ranges from roomtemperature to 200 C.

The post deposition annealing treatment with vacuum or N₂ ambient issometimes needed to improve the composition distribution of polymermaterial. The annealing temperature ranges room temperature to 300 Cwith an anneal time of less than 1 hour.

B. Spin-coat: By a spin-coater with the doped-TCNQ solution @ therotation of less than 1000 rpm. After spin-coating, the wafer is put towait the solid-state formation @ room temperature or temperature of lessthan 200 C. The waiting time ranges from several minutes to days,depending on the temperature and on the formation conditions.

FIG. 3 is a graph 300 of voltage and temperature versus time for pulsesused for programming programmable resistive memory cells, according toone embodiment of the present invention. As explained above, in phasechange memory, data is stored by causing transitions between amorphousand crystalline states in the phase change material using current.Current heats the phase change material and causes transitions betweenthe states. FIG. 3 describes the electrical pulses used to cause thetransitions. Specifically, FIG. 3 describes the bias voltages that areapplied to bit lines and word lines of a memory cell. The y-axis ofgraph 300 indicates the magnitude of the bias voltage applied to a phasechange material, as well as the temperature to which the phase changematerial is heated by each pulse. The x-axis of graph 300 indicates thepassage of time as the voltage applied to a phase change material and asthe temperature of the phase change material increases and decrease overtime,

Line 302 indicates the beginning of a RESET pulse 312, which converts aphase change material into an amorphous, high resistance state. Thechange from crystalline to amorphous, referred to as reset herein, isgenerally a higher current operation, which includes a short highcurrent density pulse to melt the crystalline structure, after which thephase change material cools quickly, quenching the phase change process,allowing at least a portion of the phase change structure to stabilizein the amorphous state. Line 304 indicates the temperature at which thephase change material transforms into an amorphous state. Note thatpulse 312 increases the temperature past line 304. The quenching phaseis indicated by the set of lines 308, wherein the phase change processthat transforms the phase change material from crystalline to amorphousis quenched or supplied with the appropriate energy.

Line 316 indicates the beginning of a SET pulse 314, which converts aphase change material into a crystalline, low resistance state. Thechange from the amorphous to the crystalline state is generally a longerpulse but a lower current operation. Line 306 indicates the temperatureat which the phase change material transforms into a crystalline state,sometimes called a breakdown transformation, which is lower than thetemperature for a amorphous state, indicated by line 304. Note thatpulse 314 increases the temperature past line 304 Also shown is interim310, which indicates the wait time that must be tolled between theactivation of SET pulse 314 after the RESET pulse 312 completes thequench process. Line 306 corresponds to the threshold voltage of thephase change elements 46-52, discussed in more detail below, above whichthe bit line voltage is set to achieve set pulse 314.

FIG. 4 is a flowchart showing the control flow of a general process forprogramming a single memory cell comprising a phase change material, inaccordance with one embodiment of the present invention. Specifically,the flowchart of FIG. 4 describes that process that takes place when acontroller, such as controller 34, programs (that is, sets or resets) asingle memory cell of a phase change memory device comprising aplurality of memory cells. FIG. 4 begins with step 402 and proceedsdirectly to step 404. In step 404, the resistance of the memory cell isread. In step 406, input data, representing the data that must bereflected by the state of the memory cell, is read. In step 408, it isdetermined whether the resistance of the memory cell is in a state thatreflects the input data. For example, if the resistance of the memorycell is high and the input data indicates a “0,” or if the memory cellis low and the input data indicates a “1,” then the resistance of thememory cell is in a state that reflects the input data. Otherwise, theresistance of the memory cell is not in a state that reflects the inputdata.

If the result of step 408 is positive, then control flows to step 410.If the result of step 408 is negative, then control flows to step 412.In step 412, the memory cell is either set or reset, depending on theprogramming process that is taking place. In step 410, the programmingprocess of the flowchart of FIG. 4 ends.

FIG. 5 is a flowchart showing the control flow of the process forprogramming a single memory cell comprising a phase change material, inaccordance with one embodiment of the present invention. Specifically,the flowchart of FIG. 5 describes that process that takes place when acontroller, such as controller 34, programs (that is, sets or resets) asingle memory cell of a phase change memory device comprising aplurality of memory cells. FIG. 5 provides more detail for the generalprocess described by the flowchart of FIG. 4.

FIG. 5 begins with step 502 and proceeds directly to step 504. In step504, the resistance of a memory cell at an address is read. In oneembodiment of the present invention, the reading step comprises applyinga voltage to a bit line coupled to the memory cell and applying avoltage to a word line coupled to an access device, the access devicebeing coupled to the memory cell. If the result of step 504 is that ahigh resistance is read, indicating the memory cell is in an at leastpartial amorphous state, then control flows to step 506. If the resultof step 504 is that a low resistance is read, indicating the memory cellis in a crystalline state, then control flows to step 508.

In step 506, input data, representing the data that must be reflected bythe state of the memory cell, is read. In one embodiment of the presentinvention, this reading step comprises receiving input data, such asfrom an input buffer, and evaluating a value of the input data. Theinput data can be, for example, a single bit value indicating a “0” or a“1.” If the result of step 506 is that a first bit value is read,indicating a “0,” then control flows to step 510. If the result of step504 is that a second bit value is read, indicating a “1,” then controlflows to step 512.

In step 512, the memory cell is set. In an embodiment of the presentinvention, the step of setting the memory cell comprises applying biasvoltages to the bit line and the word line of the memory cell so as tochange the memory cell to a crystalline, low resistance state. This steptakes place using a lower current via the bit line. This is described ingreater detail above with reference to FIG. 3.

In step 508, input data, representing the data that must be reflected bythe state of the memory cell, is read. If the result of step 508 is thata first bit value is read, indicating a “0,” then control flows to step514. If the result of step 508 is that a second bit value is read,indicating a “1,” then control flows to step 510.

In step 514, the memory cell is reset. In an embodiment of the presentinvention, the step of resetting the memory cell comprises applying biasvoltages to the bit line and the word line of the memory cell so as tochange the memory cell to an amorphous, high resistance state. This isdescribed in greater detail above with reference to FIG. 3. In step 510,the programming process of the flowchart of FIG. 5 ends.

FIG. 6 is a flowchart showing the control flow of the process forsequentially programming a plurality of memory cells comprising a phasechange material, in accordance with one embodiment of the presentinvention. Specifically, the flowchart of FIG. 6 describes that processthat takes place when a controller, such as controller 34, sequentiallyprograms (that is, sets or resets) a plurality of memory cells of aphase change memory device. The process of FIG. 6 is the extension ofthe process of FIG. 5 to a plurality of memory cells.

FIG. 6 begins with step 601 and proceeds directly to step 602. In step602, the programming sequence moves to the next available memory cellthat is slated for programming in a plurality of memory cells tat willbe programmed. In step 604, the resistance of a memory cell at anaddress is read. If the result of step 604 is that a high resistance isread, indicating the memory cell is in an at least partial amorphousstate, then control flows to step 606. If the result of step 604 is thata low resistance is read, indicating the memory cell is in a crystallinestate, then control flows to step 608.

In step 606, input data, representing the data that must be reflected bythe state of the memory cell, is read. In one embodiment of the presentinvention, this reading step comprises receiving input data, such asfrom the input buffer, and evaluating a value of the input data. Theinput data can be, for example, a single bit value indicating a “O” or a“1.” If the result of step 606 is that a first bit value is read,indicating a “0,” then control flows to step 610. If the result of step604 is that a second bit value is read, indicating a “1,” then controlflows to step 612.

In step 612, the memory cell is set. In step 608, input data,representing the data that must be reflected by the state of the memorycell, is read. If the result of step 608 is that a first bit value isread, indicating a “0,” then control flows to step 614. If the result ofstep 608 is that a second bit value is read, indicating a “1,” thencontrol flows to step 610.

In step 614, the memory cell is reset. In step 610, it is determinedwhether there are any additional memory cells that are slated forprogramming in a plurality of memory cells that will be programmed. Ifthe result of step 610 is negative, then control flows to step 616. Ifthe result of step 610 is positive, then control flows to step 602,where the process of the flowchart of FIG. 6 continues until all memorycells slated for programming have been programmed. In step 616, theprogramming process of the flowchart of FIG. 6 ends.

FIG. 7 is a flowchart showing the control flow of the process for groupprogramming of a plurality of memory cells comprising a phase changematerial, in accordance with one embodiment of the present invention.Specifically, the flowchart of FIG. 7 describes that process that takesplace when a controller, such as controller 34, programs (that is, setsor resets) a plurality of memory cells of a phase change memory deviceusing a grouping method.

FIG. 7 begins with step 702 and proceeds directly to step 704. In step704, the controller initiates a group set method wherein a set functionis sequentially applied to each memory cell in a group of memory cells.The group set method is described in greater detail below with referenceto FIG. 9. In step 706, the controller initiates a group reset methodwherein a reset function is sequentially applied to each memory cell inthe group of memory cells. The group reset method is described ingreater detail below with reference to FIG. 10. In step 708, the processof the flowchart of FIG. 7 ends.

FIG. 8 is a flowchart showing the control flow of the process for groupprogramming of a plurality of memory cells comprising a phase changematerial, in accordance with another embodiment of the presentinvention. Like the flowchart of FIG. 7, the flowchart of FIG. 8describes that process that takes place when a controller, such ascontroller 34, programs (that is, sets or resets) a plurality of memorycells of a phase change memory device using a grouping method. FIG. 8begins with step 802 and proceeds directly to step 804. In step 804, thecontroller initiates a group reset method wherein a reset function issequentially applied to each memory cell in a group of memory cells. Thegroup reset method is described in greater detail below with referenceto FIG. 10. In step 806, the controller initiates a group set methodwherein a set function is sequentially applied to each memory cell inthe group of memory cells. The group set method is described in greaterdetail below with reference to FIG. 9. In step 808, the process of theflowchart of FIG. 8 ends.

FIG. 9 is a flowchart showing the control flow of the group set processfor programming a plurality of memory cells comprising a phase changematerial, in accordance with one embodiment of the present invention.Specifically, the flowchart of FIG. 9 describes that process that takesplace when a controller, such as controller 34, sequentially applies aset process to a group of memory cells in a phase change memory device.FIG. 9 begins with step 902 and proceeds directly to step 904. In step904, the programming sequence moves to the next available memory cell inthe group of memory cells to which the group set method is beingapplied.

In step 906, the resistance of a memory cell at an address is read. Ifthe result of step 906 is that a high resistance is read, indicating thememory cell is in an at least partial amorphous state, then controlflows to step 908. If the result of step 906 is that a low resistance isread, indicating the memory cell is in a crystalline state, then controlflows to step 910.

In step 908, input data, representing the data that must be reflected bythe state of the memory cell, is read. The input data can be, forexample, a single bit value indicating a “0” or a “1.” If the result ofstep 908 is that a first bit value is read, indicating a “0,” thencontrol flows to step 910. If the result of step 908 is that a secondbit value is read, indicating a “1,” then control flows to step 912. Instep 912, the memory cell is set.

In step 910, it is determined whether the current memory cell is thelast memory cell in the group of memory cells to which the group setmethod is being applied. If the result of step 910 is negative, thencontrol flows to step 904, where the process of the flowchart of FIG. 9continues until all memory cells in the group have experienced the groupset method. If the result of step 910 is positive, then control flows tostep 914, where the group set process of the flowchart of FIG. 9 ends.

FIG. 10 is a flowchart showing the control flow of the group resetprocess for programming a plurality of memory cells comprising a phasechange material, in accordance with one embodiment of the presentinvention Specifically, the flowchart of FIG. 10 describes that processthat takes place when a controller, such as controller 34, sequentiallyapplies a reset method to a group of memory cells in a phase changememory device. FIG. 10 begins with step 1002 and proceeds directly tostep 1004. In step 1004, the programming sequence moves to the nextavailable memory cell in the group of memory cells to which the groupreset process is being applied.

In step 1006, the resistance of a memory cell at an address is read. Ifthe result of step 1006 is that a high resistance is read, indicatingthe memory cell is in an at least partial amorphous state, then controlflows to step 1010. If the result of step 1006 is that a low resistanceis read, indicating the memory cell is in a crystalline state, thencontrol flows to step 1008.

In step 1008, input data, representing the data that must be reflectedby the state of the memory cell, is read. The input data can be, forexample, a single bit value indicating a “0” or a “1.” If the result ofstep 1008 is that a first bit value is read, indicating a “0,” thencontrol flows to step 1012. If the result of step 1008 is that a secondbit value is read, indicating a “1,” then control flows to step 1010. Instep 1012, the memory cell is reset.

In step 1010, it is determined whether the current memory cell is thelast memory cell in the group of memory cells to which the group resetmethod is being applied. If the result of step 1010 is negative, thencontrol flows to step 1004, where the process of the flowchart of FIG.10 continues until all memory cells in the group have experienced thegroup reset method. If the result of step 1010 is positive, then controlflows to step 1014, where the group reset process of the flowchart ofFIG. 10 ends.

FIG. 11 is a flowchart showing the control flow of the process for resetprogramming a memory cell comprising a phase change material, inaccordance with one embodiment of the present invention. Specifically,the flowchart of FIG. 11 describes that process that takes place when acontroller, such as controller 34, reset programs a memory cell of aphase change memory device comprising a plurality of memory cells.

FIG. 11 begins with step 1102 and proceeds directly to step 1104. Instep 1104, the resistance of a memory cell at an address is read. In oneembodiment of the present invention, the reading step may compriseapplying a voltage to a word line coupled to an access device, such as apair of MOSFETs, the access device being coupled to the memory cell. Theaccess device further has a threshold voltage (referred to as “VtMOSFET”in FIG. 11).

If the result of step 1104 is that a high resistance is read, indicatingthe memory cell is in an at least partial amorphous state, then controlflows to step 1106. If the result of step 1104 is that a low resistanceis read, indicating the memory cell is in a crystalline state, thencontrol flows to step 1108. In step 1106, the reset programming processends. In step 1108, the memory cell is reset. In an embodiment of thepresent invention, the step of resetting the memory cell comprisesapplying bias voltages to the bit line and the word line of the memorycell so as to change the memory cell to an amorphous, high resistancestate. This step takes place using a higher current, which includes ashort high current density pulse to melt the crystalline structure intoan amorphous state. Specifically, the voltage applied to the bit line(referred to as “Vbl” in FIG. 11 ) is greater than the threshold voltageof the memory element (referred to as “Vtcell” in FIG. 11) and thevoltage V applied to the word line is greater than the access devicethreshold voltage.

In step 1110, the resistance of the memory cell is read again, asdescribed for step 1104 above. If the result of step 1110 is that a highresistance is read, then control flows to step 1106. If the result ofstep 1110 is that a low resistance is read, then control flows to step1112. In step 1112, the memory cell is reset once more. In this step,the voltage V is increased by an amount (referred to as “ε” in FIG. 11)and then a voltage greater than the memory element threshold voltage isapplied to the bit line and the increased voltage V is applied to theword line. Control then flows back to step 1110 where the process ofchecking the resistance of the memory cell (step 1110), increasing theword line voltage V (step 1112) and applying the voltage V to the wordline (step 1112) is executed until the resistance of the memory cell ishigh, indicating an amorphous state of the memory cell.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations will occurto those skilled in the art, which modifications and combinations willbe within the spirit of the invention and the scope of the followingclaims. Any and all patents, patent applications and printedpublications referred to above are hereby incorporated by reference.

1. A method for programming a plurality of programmable resistive memorycells, comprising: executing the following for each memory cell: (a)reading a resistance of a memory cell; (b) reading input datacorresponding to the memory cell; (c) programming the memory cell to alower resistance state if the resistance is at a higher resistance stateand the input data corresponds to a first state; and (d) programming thememory cell to a higher resistance state if the resistance is at a lowerresistance state and the input data corresponds to a second state. 2.The method according to claim 1 wherein the (c) programming step takesplace before the (d) programming step.
 3. The method according to claim1 wherein the (a) reading step comprises reading the resistance of thememory cell at an address by applying a voltage to a bit line coupled tothe memory cell and applying a voltage to a word line coupled to anaccess device, the access device being coupled to the memory cell. 4.The method according to claim 3 wherein the (b) reading step furthercomprises reading input data from an input buffer and evaluating a valueof the input data.
 5. The method according to claim 4 wherein the (c)programming step further comprises applying bias voltages to the bitline and the word line so as to change the memory cell to a generallycrystalline, low resistance state.
 6. The method according to claim 5wherein the (d) programming step further comprises applying biasvoltages to the bit line and the word line so as to change the memorycell to a generally amorphous, high resistance state.
 7. A programmableresistive memory system, comprising: a memory cell device comprising aplurality of programmable resistive memory cells; and a controller forprogramming the memory cell device, the controller configured forexecuting the following for each memory cell: (a) reading a resistanceof a memory cell; (b) reading input data corresponding to the memorycell; (c) programming the memory cell to a lower resistance state if theresistance is at a higher resistance state and the input datacorresponds to a first state; and (d) programming the memory cell to ahigher resistance state if the resistance is at a lower resistance stateand the input data corresponds to a second state.
 8. The programmableresistive memory system according to claim 7 wherein the (c) programmingstep takes place before the (d) programming step.
 9. The programmableresistive memory system according to claim 7 wherein each memory cellcomprises phase change materials.
 10. The programmable resistive memorysystem according to claim 9 wherein the phase change material comprisesGST.
 11. The programmable resistive memory system according to claim 7wherein each memory cell is coupled to a bit line and an access deviceand wherein the access device is coupled to a word line.
 12. Theprogrammable resistive memory system according to claim 1I wherein thecontroller is conductively coupled with the bit line and the word linefor each memory cell.
 13. The programmable resistive memory systemaccording to claim 12 wherein the controller comprises an energy sourcefor applying bias voltages to the bit line and the word line for eachmemory cell.
 14. A computer program product including computerinstructions for programming a memory cell device comprising a pluralityof programmable resistive memory cells, the computer instructionsincluding instructions for executing the following for each memory cell:(a) reading a resistance of a memory cell; (b) reading input datacorresponding to the memory cell; (c) programming the memory cell to alower resistance state if the resistance is at a higher resistance stateand the input data corresponds to a first state; and (d) programming thememory cell to a higher resistance state if the resistance is at a lowerresistance state and the input data corresponds to a second state. 15.The computer program product according to claim 14, wherein the (c)programming step takes place before the (d) programming step.
 16. Thecomputer program product according to claim 14, wherein the (a) readingstep comprises reading the resistance of the memory cell at an addressby applying a voltage to a bit line coupled to the memory cell andapplying a voltage to a word line coupled to an access device, theaccess device being coupled to the memory cell.
 17. The computer programproduct according to claim 16 wherein the (b) reading step comprisesreceiving input data from an input buffer and evaluating a value of theinput data.
 18. The computer program product according to claim 17wherein the (c) programming step further comprises applying biasvoltages to the bit line and the word line so as to change the memorycell to a crystalline, low resistance state.
 19. The computer programproduct according to claim 18 wherein the (d) programming step furthercomprises applying bias voltages to the bit line and the word line so asto change the memory cell to an amorphous, high resistance state.